Preheat the oven to 190C Fan.
Place a medium-sized non-stick frying pan over a medium heat. Add the chorizo (if using) and render down for a couple of minutes until crisp - you don’t need to add any oil as the chorizo has its own fat.
Remove the chorizo from the pan and place in the bowl.
If you aren't using chorizo, preheat the oven and start here. Add the butter and garlic to the pan, stir well, then add the flour and whisk to a paste. Gradually add the milk, whisking continuously to form a smooth sauce - you want it to be slightly runnier than a regular white sauce as it will thicken in the oven.
Stir through around 120g of the grated cheese. Return the chorizo to the pan, stir through, then season to taste. Remove from the heat and set aside.
Meanwhile, boil the macaroni in salted water according to packet instructions - or until al dente. Drain (reserving some of the pasta cooking water) and stir through the cheese sauce until completely coated. Taste and adjust the seasoning, as needed. If it's looking really thick you can loosen it a little with some pasta cooking water but you don't want loads - probably only 50-100ml and you may not find you need any. Tip into an ovenproof dish.
Sprinkle with the remaining grated cheese and breadcrumbs.
Pop in the oven and bake for 20 minutes - or until the top is golden and bubbling. Leave to stand for 2 minutes before serving. Enjoy!